1. Field of the Invention
This invention relates to a biaxially oriented polyester film for a magnetic recording medium. More specifically, it relates to a biaxially oriented polyester film for a magnetic recording medium, which is excellent in abrasion resistance and scratch resistance and which exhibits electromagnetic conversion characteristics when it is formed into a magnetic tape.
2. Description of the Related Art
A biaxially oriented polyester film typified by a polyethylene terephthalate film is widely used as a base film for magnetic recording media such as magnetic tape, floppy disk and the like, owing to its excellent physical and chemical properties.
In the biaxially oriented polyester film, the scratch resistance and abrasion resistance of the film and the electromagnetic conversion characteristics of a magnetic tape formed therefrom are major factors of determining the quality of workability of the film production process and fabricating process and further the quality as a product thereof.
If the slipperiness of a biaxially oriented polyester film is insufficient, wrinkles will be produced or blocking will occur in the film when it is rolled and with the result of an uneven roll surface, a reduction in production yield will follow or appropriate ranges of tension, contact pressure and speed at the time of winding up a film will be narrowed, thereby making it extremely difficult to wind up the film. Further, when the film is fabricated into a magnetic tape and the tape is caused to run in a video tape recorder, if the slipperiness of the tape is unsatisfactory, running tension will increase with the result that stopping of running will occur or debris powders will be produced with the result that omission of a magnetic recording signal, i.e., a dropout will occur. When the tape is wounded up in a magnetic tape cassette, it is loosely wound, thereby causing unstable running in the video tape recorder (VTR). Or in the case of a magnetic tape wound on a reel called "pan cake roll" before being wound up in a cassette, deformation will arise due to the loose winding.
For application in VTRs in particular, in recent years, there have been frequently seen the cases where a metal guide or plastic guide whose surface is not perfectly finished is used for a guide post fixed in a cassette to reduce costs. The surface of such guide post is extremely rough. To cope with this problem, there has been proposed a method in which fine particles having a high Mohs hardness are incorporated to the film to reduce scratches (refer to JP-A 1-306220) (the term "JP-A" as used herein means an "unexamined published Japanese patent application"). In these days, there has been known an incorporation of alumina fine particles (refer to JP-A 3-6239).
However, if the fine particles are not fully dispersed in a film, the film may not exhibit sufficient abrasion resistance or scratch resistance even in these methods.
It is therefore an object of the present invention to provide a biaxially oriented polyester film for a magnetic recording medium.
It is another object of the present invention to provide a biaxially oriented polyester film for a magnetic recording medium having excellent abrasion resistance, scratch resistance and slipperiness.
It is still another object of the present invention to provide a biaxially oriented polyester film for a magnetic recording medium, in which alumina fine particulate agglomerates or fine particulate agglomerates of a composite oxide containing alumina is surface-treated with an alkali metal salt of a polyvalent carboxylic acid so that the fine particulate agglomerates are dispersed in a polyester uniformly without growing into a coarse agglomerate in the polyester and with less increasing the degree of agglomeration.
It is a further object of the present invention to provide a biaxially oriented polyester film for a magnetic recording medium, which is excellent in film slipperiness, abrasion resistance and prevention of the surface of the film from getting scratches by a guide pin having a rough surface, i.e., scratch resistance, and further is excellent in electromagnetic conversion characteristics as a magnetic tape formed therefrom.
It is a still further object of the present invention to provide a biaxially oriented laminate polyester film for a magnetic recording medium, which has the biaxially oriented polyester film of the present invention as at least one surface layer and which has excellent slipperiness such as running durability in spite of its flat surface and has, at the same time, improved abrasion resistance, scratch resistance and electromagnetic conversion characteristics as a magnetic tape formed therefrom.
Other objects and advantages of the present invention will become apparent from the following description.